Electric filter



April 22, 1941. M F, PETERS [gl-AL 2,238,915

ELECTRIC FILTER Filed OCT.. 13, 1937 Flc-3.1 H f 2 4'lIlIlllIlllllllllllllllllllllllllllll1 V ll/ll/ l//I//l/l/Ill//l/Il/lllll FIC-3.8-

IIIIIIIIIIIIIIIIIJ ATTORN EY.

Patented Apr. 22, 1941 ELECTRIC FILTER Melville F. Peters, Beltsville,Md., and R. Harry Stone, Bound Brook, N. J., assignors to Titeflex MetalHose Co., a corporation of New Jersey Application October 13, 1937,Serial No. 168,810

\ 8 Claims.

This invention relates to electric filters whereby unwanted highfrequency current oscillations contained in circuits with lowerfrequency currents or direct current may be partially or wholly removedor filtered out.

According to a Well known phenomenon, known as skin effect, highfrequency oscillations of current are forced outwardly to the surface ofthe conductor and this is taken advantage'of to damp or separate out theundesired, high frequency oscillations whether sustained or transsient.

The invention is of great use in connection with the ignition circuitsof internal combustion engines wherein undesired transient highfrequency oscillations are set up by the rupturing of the circuitincident to such use. It is also of value in connection with thecircuits of radio apparatus wherein direct current is required andoscillations are not wanted. It is also useful for removal or reductionof transient oscillations occasioned in electric circuits operatingelevators, refrigerators and other devices. In fact the invention may beemployed Wherever undesired high frequency oscillations are to bereduced or removed from a circuit.

'I'he skin effect may be joined with the use of capacitance orinductance or ohmic resistance, any or all of them, as may be thoughtadvantageous to produce the desired results.

The invention provides a means for filtering out undesired oscillationsfrom a transmission line without increasing the resistance or inductancein the transmission line and it also has the advantage of the filterbeing small in weight which is of importance in aviation.

It also provides the advantages that the filter may'be made as a unitwhich may be inserted in and removed from a transmission line and such aunit may be of small size.

The invention also has the advantage that it may be adapted to theelimination of oscillations of different frequencies by suitablyconstructing or adjusting the filter.

One of the main objects of the invention is to provide a means foreliminating or reducing unwanted current oscillations from an electrictransmission line.

A further object of the invention is to provide an electric currentfilter having the characteristics and with some or all of the advantagesas above indicated.

A further object of the invention is to provide an improved method foreliminating or reducing unwanted electrical oscillations from atransmission line.

A further object of the invention is to provide a method of electricfiltering which shall have some or all of the advantages as aboveindicated.

Other and ancillary objects of the invention will appear hereinafter.

In the accompanying drawing which illustrates the invention- Fig.- 1 isa diagrammatic view of an ignition circuit for an internal combustionengine in which is included a current filter embodying the invention;

Fig. 2 is a longitudinal sectional View of the filter of Fig. 1, on anenlarged scale;

Figs. 3 and 4 are similar views, on the same scale as Fig. 2, ofmodified forms of filter;

Fig. 5 is a fragmentary view in longitudinal section, on an enlargedscale, of a filter similar to that shown in Fig. 1, but having a unitarystructure readily attachable to and detachable from a circuit;

Fig. 6 is a fragmentary view, on a still larger scale, of a longitudinalsection of another modified form of filter;

Figs. 7 and 8 are longitudina1 sections, showing the parts inexaggerated relationship, of a further modified form of filter; and

Fig. 9 is a diagrammatic view of an ignition circuit including thefilter of the present invention which is formed by adding parts to aprevious Well known form of current filter.

Referring to the drawing, and rst .to Figs. 1 and 2 showing theapplication of the invention t0 the ignition circuit of an internalcombustion engine, direct current is supplied from the battery I throughthe transmission conductors 2 and 3 to the -terminals of the primary 4of the step up transformer 5, the secondary 6 of which has its terminalsconnected with the terminals of the spark plug l in the cylinder of theengine. In the primary circuit is the usual switch 8 for opening andclosing the ignition circuit to place it in and out of operation. Theusual automatic interrupter contacts 9 are provided in the primarycircuit of the spark coil 5 to cause the interruptions of the directcurrent necessary to effect transmission through the spark coil ortransformer 5 and cause sparking at the spark plug gap 'l in the wellknown manner. Across the contacts 9 the condenser l0 is connected toprevent sparking at those contacts.

The interruption of the circuit at the contacts 8 and also the sparkingat fthe plug gap 1 produces transient high frequency oscillations ofelectro-motive force in the primary circuit which it is desirable toeliminate and prevent them from reaching or acting upon the battery,

To eliminate the high frequency oscillations, a filter F is placed inthe transmission conductor 2 in the primary circuit which includes thebattery. This comprises the transmission conductor II which is connectedin series in the conductor 2, the conductor II being an integral part ofthe conductor 2. Surrounding the conductor II and spaced therefrom is acylinder I2 of good electric conducting material such as copper. Thespace between the conductor II and the cylinder I2 is filled with a.poorly conducting material I3, so that it contacts with both 4thecylinder I2 and the conductor I I. The ends of the cylinder are closedby heads I4 of insulation such as Bakelite or hard rubber. Such headsmay serve to retain the material I3 within the cylinder when suchmaterial is granular or liquid or such as would be liable to run out ofthe cylinder. The heads may also engage the cylinder and thetransmission conductor I I to hold them in proper relative positions.

The cylinder I2 is connected with ground at I5. Also the primary circuiton the other side of the battery from the filter is connected to groundat I6.

When the switch 8 is closed, the well known interrupter contacts 9 willinterrupt the primary) circuit and cause the electro-motive force -to betransmitted through Ithe spark coil 5 to the spark plug gap "I where theusual sparking will take place in the cylinder of the engine, Thetransient high frequency electric oscillations occasioned by suchoperation, however, are eliminated by the nlter F. The reason for thisfiltration is that the oscillations of very high frequency are forcedoutwardly from the conductor II and across the poorly conductingmaterial I3, by the operation of skin effect, to the cylinder I2 andthence dissipated to the ground. Also the oscillations are dissipated bybeing transformed into heat in the poorly conducting material. Thegrounding at I6 also provides a shunt about the battery so that none ofthese high frequency ostion circuit illustrated will be provided withthe necessary and usual appurtenances of an ignition circuit such as atimer etc., certain of such features, however, not being necessary to anillus- 'tration of the present invention, have been omitted.

The poorly conducting material I3 may be anything suitable, eithersolid, liquid or granular. One suitable material has been found to be amixture of graphite and clay mixed in proportions to give the desiredconductivity or, reciprocally, the desired resistance. This graphite andclay may be powdered, mixed together and drypacked into the filter, orit may be bound together with sodium silicate or other suitable binder.Instead of the graphite a powdered metal, such as copper, aluminum,etc., may be mixed with the clay. Also instead of the clay, the graphiteor powdered metal might be mixed with powdered or flaked mica, Asatisfactory poorly conducting material I3 has also been formed bymixing 100 parts litharge glycerine with 30 parts of lamp black,graphite or manganese dioxide.

The poorly conducting material I3 may also consist of metallic oxides,sulphates and sulphides, which are of well known high resistances,either alone or in combination with clay, graphite or other suitablematerials, or the poorly conducting material I3 might be formed ofpowdered metals mixed with metallic oxides, sulphates or sulphideseither alone or with a suitable base to give the proper resistance.

cillations can -pass through the battery. 'I'heelectro-motive forceswhereby the operation of skin effect forces the high frequencyoscillations outwardly across the poorly conducting material to thecylinder I2, are of such value, der pending upon the frequency of theoscillations, that they force the high frequency currents across thepoorly conducting material, whereas low frequency or direct currents donot develop such high skin effect voltages and hence such currents arenot forced across the poorly conducting material or at least to only anegligible extent, battery voltage or any voltage developed in relationto the direct or low frequency current being confined within theconductor, except for a possible negligible leakage across the poorlyconducting material I3. 'I'he conducting path between the conductor IIand the cylinder I2 should therefore be made of such high resistancethat only a negligible amount of the low frequency or direct currentswhich it is desired to retain in the transmission conductor shall leakto the cylinder I2, and on the other hand the resistance of such pathshould be made such that the skin effect voltages due to the highfrequency oscillations, which it is desired to eliminate from thetransmission circuit, shall be transmitted across the poorly conductingmaterial to the cylinder and thus eliminated.

It will be understood of course that the igni- 75 The poorly conductingmaterial I3 may be a suitable liquid as for instance water, either pureor having sal-t dissolved in it until the desired conductivity isattained. l

In the ignition system of Fig. 1, an alternating current source may besubstituted for the battery, such alternating current source being of arelatively low frequency compared to the high frequency oscillations tobe eliminated by the filter, the system and filter then operating asabove described.

I n order lto shorten the filter, the wire II of Fig. 1 may be spiraledor placed in the form of a helix as shown by the transmission conductorII' of Fig. 3. This ceiling of the Wire also aids the operation of thefilter in that it provides an inductance which impedes progress of thehigh frequency oscillations along the transmission conductor.

In the modification as shown in Fig. 4, the outer cylinder I2 of Fig. 1may be formed with circumferential ridges I'I on the tube I2' toincrease the average diameter of the tube and hence increase theresistance of the path from the transmission conductor II" to the tube I2 whereby the poorly conducting material I3' may be made of greaterconductivity and still provide the necessary resistance to retain thedirect current or low frequency oscillations within the transmissionconductor, except for the negligible leakage as above referred to.

It may often prove desirable to form the filter in a unit which may bereadily inserted in or removed from any circuit, either to effectrepairs or replacements or for other purposes. A construction wherebythis may be effected is illustrated in Fig. 5 in which at each end ofthe metal cylinder I2" corresponding to the cylinder I2 of Fig. 1, thereis inserted a socket piece I8 of high grade insulation in which ismounted the metal socket piece I9 Ito which is secured in conductingrelation the transmission wire II" corresponding to the transmissionconductor Il of Fig. 1, and between which and the cylinder I2" is placedthe poorly conducting material I3" corresponding to the poorlyconducting material I3 of Fig. 1.

The metal part I9 is provided with the spring contacts 20 adapted toreceive and retain the plug 2I forming part of the metal ferrule 22fitted over the end of the insulated transmission line conductor 2'corresponding to the conductor 2 of Fig. l, the metal conductor 24 ofwhich is secured in electrical connection with the ferrule 22 by solder'g or any other suitable manner. The end, herefore, of the filter may beconnected with the transmission conductor 24 by inserting the ferrule 22within the insulating socket piece I3 and the conducting clip 2| withinthe clips or spring contacts 20. This connection to the end of thefilter with the transmission line may be obviously effected bywithdrawing the ferrule 22 from the filter. At the other end of thefilter similar provision is made for connecting and disconnecting thetransmission conductor with the filter end.

Another form of filter which may be inserted ln the transmission line asabove indicated in connection with Fig. 1, is the form shown in Fig. 6.This construction has the advantage that a considerable amount ofimpedance to the flow of high frequency oscillations in the transmissionconductor is provided by providing a yconsiderable amount of inductancein the transmission conductor. In the construction of filter as shown inFig. 6, the outer metal cylinder I2 of the filter which corresponds tothe outer cylinder I2 of the filter in Fig. 1, has within it the poorlyconducting material I3' corresponding with the poorly conductingmaterial I3 of Fig. 1, and in such material is the helically coiledtransmission conductor IIIV corresponding to the conductor II of Fig. l,but differing therefrom in that it is coiled whereby inductance isprovided in the transmission conductor which impedes the passage of theoscillations to be eliminated along such conductor and aids the skineffect in forcing such oscillations outwardly to the exterior cylinderI2"'. A support for the material I3" is provided by the cylinder 25formed of suitable insulation and the outer cylinder I2' is grounded atI5' corresponding to the grounding of the cylinder I2 at I5 in Fig. 1.

For very high frequency oscillations the interior of the cylinder 25lmay be left vacant, that is, filled with air, but where theoscillations are of sufficiently low frequency to permit of it theinductance in the coil II may be increased by placing upon the interiorof the tube or sleeve 25 the iron filings 26 which are grounded at 28,the ground connection being with a rod 21 extending for a considerabledistance through the iron fillings so as to establish good contacttherewith. These iron filings constitute a highly permeable core for themagnetic lines of force generated by the magneto-motive force of thecoiled conductor I IW and hence serve to increase the inductance of thatcoil for the purposes as indicated.

Another form of filter is shown in Fig. '7 which may be placed in atransmission conductor for the purpose of filtering out high frequencyoscillations as above described. To accomplish this the filter may beinserted in any circuit subject to have developed therein the highfrequency oscillations to be eliminated or filtered out. The filter ofFig. 7 comprises the transmission conductor 29 upon they surface ofwhich and in good electrical contact therewith is a relatively highresistance coating or sleeve 30.V The filter of Fig. 7 operates by thehigh frequency electric oscillations, tending to be transmitted alongthe conductor 29, being forced outwardly by skin effect into therelatively high resistance conducting medium, coating or sleeve 30 wherethey are dissipated or damped out by the resistance, the low frequencyor direct current remaining in the conductor 29 because of the lack oftheir development of skin effect voltages to force them intothe outerhigh resistance medium 30. A suitablematerial for the transmissionconductor 29 is the usual copper, while a suitable material for thecoating 3l)l has been found to be silver oxide.

In Fig. 8 is shown a filter of the character described in Fig. 7 havinga transmission conductor 29 of copper surrounded by a medium, coating orsleeve 30', of relatively high resistance. Such coating or sleeve 30 isgrounded at 3l so that the dissipation of the high frequencyoscillations to be filtered out, and which are forced into the outercoating or sleeve by skin effect," is not only effected by the dampingeffect of the resistance of the coating but it is also aided by theconnection of the coating torground at 3I through which suchoscillations may be dissi- Dated.

Some circuits may be already equipped with inductance coils designed todamp out the unwanted high frequency oscillations by the inductance ofthose coils. The dissipation or ridding of the circuit of suchoscillations may be greatly enhanced by the application of the filter ofthe present invention thereto.

In Fig. 9 of the drawing is shown an application of the invention whichis accomplished by adding parts to the usual arrangement of filterconsisting in a condenser or condensers connected across the two sidesof the circuit, and an inductance or inductances connected in series inthe circuit. Thus the spark plug contacts 35 in the engine cylinder arerespectively connected with the terminals of the secondary of thetransformer 36. The primary of the transformer is connected through theinterrupter contacts with one terminal of the battery 38, aspark-suppressing condenser 39 being connected about the interruptercontacts 3'I, while the other terminal of the primary is connectedthrough the induction coil 40 and the switch 4I for turning on and offthe spark, with the other terminal of the battery 38. An old form ofcurrent filter is provided in this circuit by connecting the condensers42 and 43 across the circuit, connections being established with theconductor 44 by connecting the condensers 42 and 43 with ground at 45and 46 respectively, while the conductor 44 is connected with ground at41, the inductance 40 being connected in series in the ignition circuitbetween the points of connection of the condensers 42 and 43. This oldform of filter may have features of the present invention added to itwhereby its efficiency is greatly increased by enclosing the inductance40 Within. a metal cylinder 48 having the end heads 49 of Bakelite orother suitable insulating material which serve to space the inductancecoil from the metal cylinder. The metal cylinder is lled with a suitablepoorly conducting material 50 as referred to above, such poorlyconducting material contacting with the metal of the coil 40 and withthe metal cylinder 48. The cylinder 48 is grounded at 5I. 'Ihisstructure of encased coil within a metal cylinder filled with poorlyconducting material is substantially the same as the structure shown inFig. 3, and the operation of eliminating the unwanted oscillations is ashereinbefore set forth, the operation of the old form of filter, asreferred to, being greatly enhanced. In modifying the old form of filterto include the features of the present invention, it is simply necessaryi1' the inductance coil 40 is bare, to place the cylinder 48 about itand ll in the space within the casing with the poorly conductingmaterial, the end heads, of course, being applied. li, however, the coilIII, as installed in the old f'llter, is formed of insulated wire, theimproved form tof filter may be obtained by substituting a struc'- tureas Fig. 3 for the insulated inductance coil of the old lter arrangement.While with the inductance coil having but a single layer satisfactoryresults may be produced, the desired effect is increased if the coil hasa number of layers,

of conductors.

While the invention has been illustrated in what are considered its bestapplications, it may have other embodiments Without departing from itsspirit and is not, therefore, limited to the structures shown in thedrawing.

What we claim ls:

1. The combination with a low resistance transmission conduet'or, of ahigh resistance conducting medium about said conductor into whichelectrical oscillations are adapted to be driven by skin effect, arelatively low resistance conducting material in electrical connectionwith said medium and a conductor connected with said low resistanceconducting material for conducting said electrical oscillations to adissipating medium outside the transmission line, said low resistanceconductors being continuously in contact with said medium.

2. The combination with a low resistance transmission conductor, of ahigh resistance conducting medium about said conductor into whichelectrical oscillations are adapted to be driven by skin effect, arelatively low resistance conducting material in electrical connectionwith said medium and a connection for conducting the electricaloscillations from said last mentioned conductor of low resistance, saidlow resistance conductors being continuously in contact with said mediumand a low resistance conductor connected with said low resistanceconducting material for conducting the said electrical oscillations to adissipating medium outside the transmission line.

3. The combination with a low resistance transmission conductor, of ahigh resistance conducting mediumabout said conductor into whichelectrical oscillations are adapted to be driven by skin effect, arelatively low resistance conducting material in electrical connectionwith said medium and a ground connection for conducting the electricaloscillations from said last mentioned conductor of low resistance, saidlow resistance conductors being continuously in contact with saidmedium.

4. The combination with a transmission line adapted to have generated init currents of dif- CII ferent frequencies, of a. high resistanceconducting mediumabout said transmission line into which electricaloscillations are adapted to be driven by skin effect," a. relatively lowresistance conducting material in electrical connection with said mediumand a. connection for conducting the electrical oscillations from saidlast mentioned conductor of low resistance to a dissipating mediumoutside the transmission line, said resistance being of a value toreduce any leakage of current desired to be retained in the transmissionconductor to a negligible amount but to permit the passage of the highfrequency currents to be eliminated, said transmission line and said lowresistance conducting material being continuously in contact with saidmedium.

5. The combination with a transmission line adapted to have generated init a direct current and an oscillatory current, of a high resistanceconducting medium about said transmission line into which electricaloscillations are adapted to be driven by skin effect, a relatively lowresistance conducting material in electrical connection with said mediumand a connection for conducting the electrical oscillations from saidlast mentioned conductor of low resistance to a dissipating medium, saidresistance being of a value to permit the passage of the oscillatorycurrents but to reduce the direct current flow therethrough to anegligible amount, said transmission line and said low resistanceconducting material being continuously in contact with said medium.

6. The combination with a low resistance transmission conductor, of alow resistance conducting tube surrounding said conductor but spacedtherefrom, and a high resistance conducting material in the spacebetween said conductor and said tube and in electrical connection withboth said conductor and said tube, said transmission conductor beingadapted to have impressed upon it oscillatory currents which are forcedacross the said high resistance material to said tube by skin effect,said low resistance transmission conductor and said tube beingcontinuously in contact with said material, and a connection forconducting the electrical oscillations from said tube to a dissipatingmedium loutside the transmission line.

7. The combination as specified in claim 6 but with the path across saidhigh resistance material being such that the high frequency oscillationsdesired to be eliminated will be forced thereacross by skin effect butlow frequency currents desired to be retained in the transmissionconductor will not be forced across said material more than a negligibleamount.

8. The combination as specified in claim 6 but with the path across saidhigh resistance material being such that the high frequency oscillationsdesired to be eliminated will be forced thereacross by skin effect butdirect current desired to be retained in the transmission conductor willpass over said path to not more than a negligible extent.

R. HARRY STONE.

MELVILLE F. PETERS.

